The present invention relates to improvements in paper making machines, and more particularly to an improved press section for pressing water from multi-ply board, and more particularly to a press section having a plurality of double felted nips arranged to dewater multi-ply board without disturbing the ply bond between the layers.
In the formation of multi-ply board of the type used for products such as soapboxes, food packages, cereal boxes and shoe boxes, the board or web is formed in individual plies with sequential plies laid on top of each ply until the desired number of plies are formed. The multi-ply board is then passed through a series of press nips for further dewatering before traveling to a dryer section. In the formation section of the machine where the layers are placed one on top of the other, dewatering usually has to be done through a number of plies to that the board is relatively wet as it passes through the press section. The interlock between the plies is formed by the fibers of paper stock, and this is known as "ply bond". It is essential that this orientation between the fibers which forms the ply bond not be disturbed to maintain maximum ply bond for strength of the board. Substantial care is taken and machine formation is particularly directed at obtaining an improved ply bond in the forming section so that this should not be reduced and instead, if possible, it should be enhanced in the press section.
It has been discovered that the fiber orientation which creates the ply bond is damaged and the strength of the ply bond reduced by rapid dewatering or crushing. This crushing is caused by the phenomena of hydraulic pressure forming within the interstices between the fibers if the water is caused to rush out too fast from high pressure pressing in the press section. In single ply webs the crushing disorients the fibers and damages the paper, and in multi-ply board, this damage also manifests itself in the destruction of the ply bond. It is, therefore, essential in removing the substantial quantities of water which must be removed in multi-ply board that crushing does not occur to reduce the strength of the ply bond.
It has also been discovered that if the multi-ply web is carried around a roll between two felts, because the outer felt must travel over a longer distance than the inner felt, a shear is induced on the web tending to separate the plies. In other words the surface of the outer felt which is in engagement with the wet web must travel over a longer distance than the surface of the inner felt which is in contact with the other surface of the web so that a shifting or shear stress is induced in the web tending to force the outer surface in one direction and the inner surface in another direction which separates the fragilly united fibers forming the ply bond. Yet, it is desirable for the sake of applying higher nip pressures to carry the web between two felts in passing through the nip. The present invention contemplates providing a press which utilizes the advantages of a double felted press nip without the disadvantages of the phenomena of ply bond destruction.
It is accordingly an object of the present invention to provide an improved press section for a multi-ply board making machine capable of dewatering a multi-ply web without disturbing the bond between the plies.
A further object of the present invention is to provide an improved section of a multi-ply paper making machine which is capable of removing substantial amounts of water from the web without causing crushing and consequent disturbance of ply bonding.
A further object of the invention is to provide an improved press arrangement particularly well suited for multi-ply web which is capable of successful and improved dewatering at high speeds for board making and which avoids the disadvantages of crushing and loosening of ply bonding.
Other objects, advantages and features will become more apparent, as will equivalent structures and methods which are intended to be covered herein, with the teaching with the principles of the invention in connection with the disclosure of the preferred embodiment in the specification, claims and drawings, in which: